Electrostatic printing apparatus



y 19, 1954 P. E. WRIGHT 3,133,484

ELECTROSTATIC PRINTING APPARATUS Filed Sept. 29, 1961 2 Sheets-Sheet l INVENTOR. 1941/1 5 WI/G'f/f May 19, 1964 P. E. WRIGHT 3,133,434

ELECTROSTATIC PRINTING APPARATUS Filed Sept. 29, 1961 2 Sheets-Sheet 2 z-r mliu r INVENTOR. FAA/A 5 FM/aw y;fm

United States Patent 3,133,484 ELECTROSTATIC PRINTING APPARATUS Paul E. Wright, Haddonfield, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Sept. 29, 1961, Ser. No. 141,666 9 Claims. (Cl. 951.7)

This invention relates generally to the art of electrostatic printing, and more particularly to improved apparatus and a method for developing a latent electrostatic image on an electrophotographic recording element.

In most prior art electrostatic printing processes where in a latent electrostatic image is produced on a recording element, the latent electrostatic image is developed by a developer that is in either a liquid or a powdered form. While liquid or powder developers present no serious handling problems to skilled operators, these developers may present problems of spilling, staining, and undesirable scattering in the hands of an unskilled operator. Images developed by powder or liquid developers are usually fixed by heat or chemical means that may produce obnoxious fumes. Also, the use of liquid developers in certain prior art apparatus often limits the position of the recording element with respect to the developer during the process of development because the recording element must pass through or near a trough of the liquid developer usually in a substantially horizontal position.

It is an object of the present invention to provide im proved developing apparatus employing a meltable, or fusible, developer that is originally in a solid form so as to eliminate the handling problems presented by liquid and powdered developers.

Another object of the present invention is to provide an improved method of applying a fused developer in controlled amounts to a recording element.

Still another object of the present invention is to provide improved developing apparatus adapted to utilize a fusable developer, originally in a solid form, in a manner whereby the fused developer may be applied to a recording element at any desired angle with respect to the horizontal.

A further object of the present invention is to provide improved developing apparatus that utilizes developers that are self-fixing.

Still a further object of the present invention is to provide improved developing apparatus that is relatively simple in structure, easy to operate, and highly efiicient in use.

Briefly, the improved developing apparatus, in accordance with the present invention, comprises means to melt a fusable developer and means to force the fused developer through a porous or apertured plate for application to a latent image on a recording element. In one embodiment of the invention, the porous plate forms one of the walls of a developer chamber. The porous plate is electrically conductive and is connected in an electrical circuit so as to melt solid developer at the interface between the plate and the solid developer. Pressure is exerted on the developer to force the melted developer through the porous plate for application to the recording element. In another embodiment of the invention, a heating coil is imbedded within the porous plate to heat the latter. In still another embodiment of the invention, the porous plate is heated by inductive means. The inductive means may also form part of a cooling system to cool the porous plate when it is desired to solidify the melted developer.

The novel method of the present invention comprises the steps of melting a solid developer and applying the melted developer to a latent electrostatic image to develop it.

Other novel features of the present invention, both as to its organization and methods of operation, as well as additional objects and advantages thereof, will be described in greater detail by reference to the accompanying drawings, in which similar reference characters designate similar parts thoughout, and in which:

FIG. 1 is a perspective view of an electrostatic printing system, with parts broken away, showing parts of the improved developing apparatus of the present invention;

FIG. 2 is a cross-sectional view of one embodiment of the improved developing apparatus taken along the line 2-2 of FIG. 3;

FIG. 3 is a cross-sectional View of the improved developing apparatus of FIG. 2, taken along the line 3-3 of FIG. 2;

FIG. 4 is a cross-sectional view of another embodiment of the present invention taken along the line 44 of FIG. 5;

FIG. 5 is a cross-sectional view of the improved developing apparatus shown in FIG. 4, taken along the line of 5-5 of FIG. 4;

FIG. 6 is a cross-sectional view of still another embodiment of the developing apparatus of the present invention taken along the line 66 of FIG. 7; and

FIG. 7 is a cross-sectional view of the improved developing apparatus shown in FIG. 6, taken along the line 7-7 of FIG. 6.

Referring now to FIG. 1, there is shown a continuous electrostatic printing system 10 for exposing an electrophotographic element 12 to produce an electrostatic image thereon and for developing the electrostatic image. The recording element 12 is in the form of a continuous web and may comprise a backing sheet 14, such as paper, and a photoconductive layer 16. The photoconductive layer 16 may comprise photoconductive zinc oxide imbedded in a resin binder, in a manner known in the art. Suitable recording elements are described in an article, Electrofax Direct Electrophotographic Printing on Paper, by C. J. Young and H. G. Greig, in RCA Review, December 1954, vol. XV, No. 4. The recording element 12 is drawn from a roll 18 and directed between electrostatic charging devices 20 and 22. The charging devices 20 and 22 are connected to the negative and positive terminals, respectively, of a unidirectional high-voltage source 23 to apply a uniform electrostatic charge to the photoconductive layer 16. The recording element 12 is directed around an air bearing 24 and between a pair of rollers 26 and 28. The roller 28 is coupled to a motor 30 to move the recording element 12 in the direction indicated by the arrows 32 and 34.

The charged photoconductive layer 16 may be exposed with a projected image from a slide 36 by means of light from a slide projector 38. Such an exposure produces a latent electrostatic image on the charged photoconductive layer 16.

The latent electrostatic image can be developed by a developer of toner particles that adhere to it. The developer in the device of FIG. 1 is originally in the form of one or more solid blocks that are adapted to melt when heat is applied to them. Blocks of solid developer 74 are enclosed within novel developing apparatus 40, which will be described hereinafter in greater detail.

The developing station, that is, the place Where the latent image on the recording element 12 is developed, is between the air bearing 24 and the developing apparatus 40. The recording element 12, in passing through the developing station, has applied to it melted developer for developing the latent image thereon, in a manner to be hereinafter described.

Solid meltable developer suitable for use in the novel developing apparatus 40 may consist essentially, for example, of up to about 15 percent by Weight of a finelydivided electroscopic toner uniformly dispersed in a thermoplastic, electrically-insulating carrier. The carrier should preferably have a melting point in a range from about 50 C. to 200 C. The molten developer should have a viscosity that is not in excess of 400 centipoises. Suitable electroscopic developer toners may be, for example, carbon black, rose bengal, benzidine yellow, and the like. Suitable thermoplastic carriers may comprise, for example, paraiiin wax (melting point 55 C. to 79 C.), carnauba wax (melting point about 84 C.), gum rosin (melting point about 100 C. to 140 C.), or the like, or suitable mixtures thereof.

A suitable solid developer for use in the novel develop ing apparatus 40 may, for example, consist essentially of a mixture of finely-divided carbon black particles dispersed in a melt of paraflin wax in proportions of about 5 parts by weight of carbon black to 95 parts by weight of wax. The mixture is stirred continuously until a uniform dispersion is obtained. The melt is allowed to cool to form a solid block of developer. When remelted and applied to a surface bearing a latent electrostatic image, the molten developer functions to develop the latent image as a result of the electrophoretic deposition of the carbon black particles in the charged areas of the latent image. When dry, the developed image is fixed, no further heating being necessary.

Referring now to FIGS. 2 and 3, there is shown one example of developing apparatus 40 which may be employed in the system of FIG. 1. The developing apparatus 40 comprises a developer chamber 42 defined by a bottom wall 44, side walls 46, 48, 50, and 52, and a top wall, the latter top wall being in the form of a plate 54 of porous material. The porous plate 54 comprises electrically conductive material that is insulated from its surrounding walls 46, 48, 50, and 52 by any suitable means (not shown). A pipe 56 extends through the bottom wall 44 and communicates with the chamber 42 for applying air under pressure to the chamber. Solid developer 74 of the type adapted to melt at a temperature above normal room temperature is disposed within the chamber 42 through any suitable opening therein (not shown).

The porous plate 54 may comprise a sintered metal of bronze, aluminum, steel, or steel alloy, for example. In the formation of these porous materials, beads of these metals are sintered to provide plates of porosities varying from 5 percent to 80 percent, depending upon the size of the beads. Sintered bronze material, having a porosity of 25 percent, in the form of degreased Oilite, a product of the Chrysler Corporation, has been found suitable. Solid plates formed with a plurality of holes of suitable size, or screens of suitable mesh, may also be employed for the porous plate 54. The preferred porosity of the porous plate 54 is determined by the size of the developer particles to be accommodated, the viscosity of the molten developer, and the amount of pressure available to force the molten developer through the porous plate 54. A porous plate 54 suitable for many of the applications of the present invention may have a porosity that permits a chosen molten developer to penetrate the plate with a differential pressure of only a few pounds per square inch across the porous plate 54.

The air bearing 24 comprises a chamber 58 defined by a top wall 60, side walls 62, 64, 66, and 68, and a bottom wall, the latter bottom Wall being in the form of a plate 70 of porous material. A tube '72 extends through the wall 60 and communicates with the chamber 58. The tube 72 serves to apply air under pressure to the chamber 58. The porous plates 54 and 70 are parallel to each other and spaced apart from each other so that the recording element 12 may pass therebetween for developing purposes. Air under pressure may be supplied to the chambers 42 and 58 from a pump 73, as shown in FIG. 1.

Means are provided to heat meltable solid developer 74 of the type described within the chamber 42. To this 4 end, the porous plate 54 is connected in an electrical chcuit of a suitable voltage source 76 and a variable resistor '78 to send heating current through the porous plate 54. The amount ofcurrent passing through the plate 54 is controlled by the variable resistor 78.

In operation, the recording element 12, with its latent electrostatic image thereon, is passed through the developing station between the air bearing 24 and the developing apparatus 40 with the latent image facing the developing apparatus. Air pressure applied through the tube 72 forces the recording element 12 adjacent to the outer surface of the porous plate 54. Current flowing through the porous plate 54 melts the solid developer 74 at the interface 75 between the porous plate 54 and the solid developer 74. Air pressure applied through the tube 56 forces the melted developer 74 through the porous plate 54 and onto the photoconductive layer 16 of the recording element 12 to develop the latent electrostatic image thereon. The air bearing 24 should force the recording element 12 to a position whereby the melted developer forms a thin liquid film between the porous plate 54 and the recording element 12.

Another arrangement 40a of the developing apparatus of the system of FIG. 1 is shown in FIGS. 4 and 5. The structure of the developing apparatus 40a is substantially similar to that of the developing apparatus 40 of FIGS. 2 and 3, but differs in that a heating element 82 is used to heat the porous plate 54 instead of relying upon the electrical resistance of the plate 54. The heating element 82 is connected in circuit with the voltage source 76 and the variable resistor 78. The heating element 82 must be electrically insulated from the porous plate 54 where the latter is electrically conductive. In the embodiment 40a, however, the porous plate 54 may comprise electrically insulating material, such as porous alumina. The porous plate 54 may also comprise a screen, of suitable mesh and electrical resistance, that has been flame sprayed with aluminum oxide. The heating current to the heating element 82 is controlled by the variable resistor 78. The operation of the embodiment 40a is the same as that explained for the embodiment 40.

In FIGS. 6 and 7, there is shown an embodiment 40b of the developing apparatus for use in the system of FIG. 1 wherein the porous plate 54 of electrically conductive material is heated by inductive means. The porous plate 64 is surrounded by a coil 84 of hollow pipe. The ends of the coil 84 are connected to output terminals of a radiofrequency generator 86. The input terminals of the generator 86 are connected to a suitable operating voltage source 77 through a switch 88. The switch 88 is also connected to a pump 90 so as to turn the pump 90 on when the generator 86 is turned off, and vice versa. When the switch 88 turns the radio-frequency generator on, the resultant high-frequency magnetic field about the coil 84 causes the porous plate 54 to become heated sufilciently, by induction heating, to melt the solid developer 74 at the interface '75 between the developer 74 and the porous plate 54. With the exception of the heating means for the porous plate 54, the operation of the embodiment 40b for developing a latent image on the recording element 12 is the same as that described for the embodiment 40 of the invention.

It is sometimes desirable to solidify the melted developer 74 quickly when development of a recording element 12 is not taking place. To this end, means are provided to cool the porous plate 54 quickly. The ends of a hollow coil 92 are connected to the ends of the coil 84 so that a coolant may be circulated through the coil 84. Thus, one end of the cooling coil 92 is connected to one end of the coil 84 through the pump 90 and a tubular electrical insulator 94. The other end of the cooling coil 92 is connected to the other end of the coil 84 through a tubular electrical insulator 96. When it is desirable to cool the porous plate 54, the switch 88 is moved to shut the radiofrequency generator off and to turn the circulating pump 90 on. Since the coil 92 is disposed within a refrigerator 98, a coolant within the cooling coil 92 can be circulated, via the pump 90, through the coil 84 to cool the coil 84 quickly.

From the foregoing description, it will be apparent that there has been provided novel developing apparatus for utilizing solid developer in an electrostatic printing process. While some of the apparatus has been shown in diagramatic form, variations in the apparatus itself coming within the spirit of this invention will, no doubt, readily suggest themselves to those skilled in the art. Hence, it is desired that the foregoing description of the invention shall be considered merely as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for developing a latent electrostatic image on a recording element, said apparatus comprising a plate of porous material, means to dispose a meltable developer on one side of said plate, means to heat said plate to a temperature to melt said developer, means to support said recording element for development with said electrostatic image close to the other side of said plate and means to force said developer in a melted state through said plate to the other side thereof for application to said latent image, whereby to develop said latent image.

2. Apparatus for developing a latent electrostatic image on a recording element, said apparatus comprising a plate of electrically conductive porous material, means to dispose a meltable developer in a solid state on one side of said plate, means connecting said plate in an electrical circuit to heat said plate to a temperature suflicient to melt said developer when in contact therewith, means to support said recording element for development with said electrostatic image close to the other side of said plate and means to force said developer while molten through said plate to the other side thereof for application to said latent image, to develop said latent image.

3. Apparatus for developing a latent electrostatic image on a recording element, said apparatus comprising a plate of porous material, means to dispose meltable developer in a solid state on one side of said plate, means including a heating element within said plate to heat said plate to a temperature sufficient to melt said developer when in contact therewith, means to support said recording element for development with said electrostatic image close to the other side of said plate, and means to force said developer while melted through said plate to the other side thereof for application to said latent image, to develop said latent image.

4. Apparatus for developing a latent electrostatic image on a recording element, said apparatus comprising a plate of electrically conductive porous material, means to dispose meltable developer in a solid state on one side of said plate, means to heat said plate to a temperature to melt said developer means to support said recording element for development with said electrostatic image close to the other side of said plate, means to force said developer while melted through said plate to the other side thereof for application to said latent image, whereby to develop said latent image, said heating means comprising a coil in an inductive relationship with said plate, and means connected to said coil to produce electrical oscillations therein whereby to heat said plate inductively.

5. Apparatus for developing a latent electrostatic image on a recording element, said apparatus comprising a plate of electrically conductive porous material, means to dispose meltable developer in a solid state on one side of said plate, means to heat said plate to a temperature sufiicient to melt said developer, means to support said recording element for development with said electrostatic image close to the other side of said plate, means to force said developer while melted through said plate to the other side thereof for application to said latent image, whereby to develop said latent image, said heating means comprising a coil of hollow tubing adjacent to said plate, means 6 connecting said coil to a source of radio-frequency current to heat said plate inductively through said coil, a refrigerant, means to cool said refrigerant, and means to circulate said refrigerant through said hollow coil when it is desired to cool said plate.

6. In apparatus of the type for making an electrostatic print on an electrophotographic recording element, said element comprising a backing sheet and a photoconductive layer, means to form an electrostatic latent image on said photoconductive layer, a developing station, and means to move said latent image along a path through said developing station, the improvement comprising means included in said station to develop said latent image, said developing means comprising a plate of porous material having one side adjacent to the path of movement of said latent image, means to dispose solid meltable developer on the other side of said plate, means to heat said plate to a temperature to melt said developer when in contact therewith, and means to apply pressure on said developer to force melted developer through said plate and in contact with said latent image, whereby to develop said latent image.

7. In apparatus of the type for making an electrostatic print on an electrophotographic recording element, said element comprising a backing sheet and a photoconductive layer, means to charge said layer with an electrostatic charge, means to expose said charged layer with a light image to produce an electrostatic latent image thereon, a developing station, and means to move said latent image along a path through said developing station, the improvement comprising means included in said station to develop said latent image, said developing means comprising a plate of porous material having one side adjacent to the path of movement of said latent image, means to dispose solid meltable developer on the other side of said plate, means including a heating element disposed within said plate to heat said plate to a temperature to melt said developer when in contact therewith, and means to apply pressure on said developer to force melted developer through said plate and in contact with said latent image, whereby to develop said latent image.

8. In apparatus of the type for making an electrostatic print on an electrophotographic recording element, said element comprising a backing sheet and a photoconductive layer, means to charge said layer with an electrostatic charge, means to expose said charged layer with a light image to produce an electrostatic latent image thereon, a developing station, and means to move said latent image along a path through said developing station, the improvement comprising means included in said station to develop said latent image, said developing means comprising a plate of porous material having one side adjacent to the path of movement of said latent image, means to dispose solid meltable developer on the other side of said plate, means to heat said plate by induction to a temperature to melt said developer when in contact therewith, and means to apply pressure on said developer to force melted developer through said plate and in contact with said latent image, whereby to develop said latent image, said induction heating means comprising a coil adjacent to said plate, and means to connect said coil to a source of radiofrequency energy.

9. In apparatus of the type for making an electrostatic print on an electrophotographic recording element, said element comprising a backing sheet and a photoconductive layer, means to charge said layer with an electrostatic charge, means to expose said charged layer with a light image to produce an electrostatic latent image thereon, a developing station, and means to move said latent image along a path through said developing station, the improvement comprising means included in said station to develop said latent image, said developing means comprising a plate of porous material having one side adjacent to the References Cited in the file of this patent path of movement of said latent image, means to dispose UNITED STATES PATENTS solid meltable developer on the other side of said plate, 453,090 Newman May 26 1891 means to heat said plate to a temperature to melt said de- 1,122,473 Butler Dec 29 1914 veloper when in contact therewith, and means to app y 5 2,297,691 Carlson Oct. 6, 1942 pressure on said developer to force melted developer 2,468,922 Cook May 3, 1949 through said plate and in contact with said latent image, 2,497,665 y 19 whereby to develop said latent image, said heating means V {D 1 5 comprising a coil of tubing adjacent to said plate, means 10 2843721 s? 1958 connecting said coil to a source of radio-frequency energy, 2:875:556 g Mar. 1959 a coolant fluid, and means to circulate said coolant fluid 2,879,397 Lehmann Mar. 1959 through said coil of tubing- 3,032,635 Kraft May 1, 1962 

7. IN APPARTUS OF THE TYPE FOR MAKING AN ELECTROSTATIC PRINT ON AN ELECTROPHOTOGRAPHIC RECORDING ELEMENT, SAID ELEMENT COMPRISING A BACKING SHEET AND A PHOTOCONDUCTIVE LAYER, MEANS TO CHARGE SAID LAYER WITH AN ELECTROSTATIC CHARGE, MEANS TO EXPOSE SAID CHARGED LAYER WITH A LIGHT IMAGE TO PRODUCE AN ELECTROSTATIC LATENT IMAGE THEREON, A DEVELOPING STATION, AND MEANS TO MOVE SAID LATENT IMAGE ALONG A PATH THROUGH SAID DEVELOPING STATION, THE IMPROVEMENT COMPRISING MEANS INCLUDED IN SAID STATION TO DEVELOP SAID LATENT IMAGE, SAID DEVELOPING MEANS COMPRISING A PLATE OF POROUS MATERIAL HAVING ONE SIDE ADJACENT TO THE PATH OF MOVEMENT OF SAID LATENT IMAGE, MEANS TO DISPOSE SOLID MELTABLE DEVELOPER ON THE OTHER SIDE OF SAID PLATE, MEANS INCLUDING A HEATING ELEMENT DISPOSED WITHIN SAID PLATE TO HEAT SAID PLATE TO A TEMPERATURE TO MELT SAID DEVELOPER WHEN IN CONTACT WTHERWITH, AND MEANS TO APPLY PRESSURE ON SAID DEVELOPER TO FORCE MELTED DEVELOPER THROUGH SAID PLATE AND IN CONTACT WITH SAID LATENT IMAGE, WHEREBY TO DEVELOP SAID LATENT IMAGE. 